WASHINGTON - Hillary Clinton came under relentless fire last night from fellow Democrats, who slammed her on issues ranging from Iran to Social Security, and all but called their rival a liar as they sought to slow down the New York senator's campaign momentum.

Senator Barack Obama of Illinois said Clinton shifted her positions on the Iraq war and the North American Free Trade Agreement. Former senator John Edwards of North Carolina accused her of supporting a "broken system" in Washington and of enabling President Bush to advance toward war with Iran. And Senator Chris Dodd of Connecticut, who has had negligible showings in the polls, questioned whether his New York colleague was electable.

At the Democratic debate at Drexel University in Philadelphia, Clinton fielded repeated shots from her rivals, including three current Senate colleagues, who are competing their party's nomination.

"Senator Clinton . . . has been for NAFTA, now she's against it. She voted for a way to authorize sending troops to Iraq, and later said this was a war for diplomacy," Obama said in his opening salvo. "That may be politically savvy, but I don't think it offers the clear contrast we need."

When Clinton refused to detail her plans for Social Security or to say whether she would support giving driver's licenses to illegal immigrants - as Governor Elliot Spitzer of New York is trying to do - her opponents accused her of dodging the issues.

"In case I misheard, Senator Clinton said two different things in the last two minutes," Edwards said about the driver's license issue.

Clinton called the driver's license issue a "gotcha" question. But her response, late in the debate, provided ammunition to her opponents, who have accused the New York senator of sidestepping questions to protect herself politically.

Obama shook his head theatrically when Clinton - pressed several times to say whether she agreed with the position of her state's governor - refused to state a clear view. "I am confused by Senator Clinton's answer. I can't tell if she is for it or against it," Obama said, adding that he thinks the driver's licenses should be provided to undocumented immigrants so that drivers can be identified and tracked in case of a car crash.

Clinton, clearly anticipating the attacks after reports this week that Obama was planning to be tougher on her, spoke calmly and evenly as she sought to deflect the criticisms, expressing disbelief that she could be considered insufficiently critical of Bush and Republicans.

"I don't think Republicans got the message that I'm voting and sounding like them," Clinton said. "I have stood against George Bush and his failed policies."

Clinton found herself under attack on her credibility - much as her husband, Bill Clinton, was in 1992 when his opponents dubbed him "Slick Willy" for what they called his shifting stands on issues.

Her Democratic foes posited that Republicans were attacking her because they thought she would be the easiest candidate to defeat next year. "Part of the reason the Republicans are obsessed with you is that is a fight they are willing to be having," Obama said.

Typical of leading candidates, Clinton did not launch a counterattack against her Democratic opponents for the nomination, saving her shots for Bush and congressional Republicans. She said they had short-changed American children on healthcare and were bent on privatizing Social Security. All the Democrats on the stage last night are in favor of expanding the State Children's Health Insurance Program to provide health insurance for low- and middle-income children, and none backs the privatization of the country's popular retirement program.

In addition to the heated exchange over her position on driver's licenses for immigrants and despite aggressive questioning by both the debate's moderators and her fellow presidential contenders, Clinton also would not agree to push for the release of White House memos between her and her husband. She said it was in the hands of the National Archives.

But the New York senator was forced to defend some of her votes and positions, including her vote for a nonbinding resolution deeming the Iranian Revolutionary Guard a terrorist organization. Clinton is the only Democratic candidate who supported the resolution; she said it bolstered diplomatic tools to deal with Iran, such as economic sanctions, but her opponents said it could lead the nation into another war in the Middle East.

"I think it can be used as a de facto declaration" of war, said Senator Joseph R. Biden Jr. of Delaware, chairman of the Senate Foreign Relations Committee, chiding Clinton for a vote that he said would embolden Republican hard-liners. "There are consequences for what we do."

Dodd likened the decision to her 2002 vote to authorize force in Iraq. "I believe that this . . . is going to come back to haunt us," Dodd said. "I'm very concerned we're going to see those 78 votes [for the Iran resolution] come back, waved in our face." Governor Bill Richardson of New Mexico also expressed his concerns about the Iran vote, although he decried what he called the personal nature of the criticism of Clinton.

Clinton said she is "not in favor of a rush to war" with Iran, but added, "I also am not in favor of doing nothing." She declined to say where she would draw the "red line," at which she would consider an attack on Iran. The Bush administration suspects Iran is trying to develop a nuclear weapon.

The candidates saved a few shots for the Republican contenders, with Biden saying former New York mayor Rudy Giuliani was the least qualified GOP candidate for president since George Bush. "There's only three things he mentions in a sentence - a noun, a verb, and 9/11, Biden said, drawing raucous laughter from the crowd.

And Obama, asked how he would dress for Halloween, suggested he might go as the former governor of Massachusetts. "I am thinking of wearing a Mitt Romney mask, but it has two sides to it," Obama quipped, referring to allegations that Romney has flip-flopped on issues.